A power converter is a power processing circuit that converts an input voltage waveform into a specified output voltage waveform. In many applications requiring a DC output, switched-mode DC-DC converters are frequently employed to advantage. DC-DC converters generally include an inverter, a transformer having a primary winding coupled to the inverter and a rectifier coupled to a secondary winding of the transformer. The inverter generally includes a switching device, such as a field-effect transistor (FET), that converts the DC input voltage to an AC voltage. The transformer then transforms the AC voltage to another value and the rectifier generates the desired DC voltage at the output of the DC-DC converter.
Conventionally, the rectifier includes passive rectifying devices, such as Schottky diodes, that conduct the load current only when forward-biased in response to the input waveform to the rectifier. Passive rectifying devices, however, cannot achieve forward voltage drops of less than about 0.35 volts, thereby substantially limiting a conversion efficiency of the DC-DC converter. To achieve an acceptable level of efficiency, DC-DC converters that provide low output voltages (e.g., 1 volt) often require rectifying devices that have forward voltage drops of less than about 0.1 volts. The DC-DC converters, therefore, generally use synchronous rectifiers. A synchronous rectifier replaces the passive rectifying devices of the conventional rectifier with an active rectifier circuit having first and second rectifier switches, such as FETs or other controllable switches, that are periodically driven into conduction and non-conduction modes in synchronism with the periodic waveform of the AC voltage. The rectifier switches of the active rectifier circuit exhibit resistive-conductive properties and may thereby avoid the higher forward voltage drops inherent in the passive rectifyig devices.
One difficulty with using a rectifier switch (e.g., an n-channel silicon FET) is the need to provide a drive signal that alternates between a positive voltage to drive the rectifier switch into a conduction mode and a zero or negative voltage to drive the rectifier switch into a non-conduction mode. Although a capacitive charge within the rectifier switch may only be 30 to 50 nanocoulombs, the rectifier switch requires a high drive current for a brief period of time to change conduction modes. Typical drive currents may be 10 amperes or greater, lasting for tens of nanoseconds. The need to provide substantial power to the rectifier switch to change conduction modes thus reduces some of the advantages of the synchronous rectifier.
The '138 patent, the '482 patent and the '541 patent all describe the use of the secondary winding of the transformer to directly drive the synchronous rectifier. The recognition of the availability of suitable drive voltages from the secondary winding over the entire switching cycle of the inverter led to the development of self-synchronized synchronous rectifiers as disclosed in the aforementioned patents.
The '032 patent describes the use of extra windings in the transformer and voltage-limiting switches to improve the control of the drive signals. The extra windings are particularly useful when the output voltage is so low that the secondary winding does not develop sufficient voltage to ensure that the rectifier switches are fully driven into the conduction mode. The voltage-limiting switches are useful when the input or output voltages are variable, resulting in wide voltage variations in the drive signals. The extra windings and voltage-limiting switches thus allow the transformer to provide drive signals of sufficient voltage to more efficiently operate the synchronous rectifier.
While the rectifier switch avoids the higher forward voltage drops inherent in passive rectifying devices, the rectifier switch possesses a number of power loss mechanisms. During switching transitions, the body diode of the rectifier switch exhibits conduction losses. Switching losses are also associated with charging and discharging of the parasitic capacitances of the rectifier switch and with reverse recovery of the body diode. Because the input capacitance of a rectifier switch may be its largest parasitic capacitance, the loss associated with charging the input capacitance may be greater than other switching losses.
When the switching frequency of a DC-DC converter is increased to achieve a more compact design, the energy required to charge and discharge the input capacitance of the rectifier switch can result in substantial losses, detracting from, and ultimately limiting, the benefits of the low conduction mode resistance of the rectifier switch. In contrast to the conduction loss, which varies with the square of the load level, the switching loss is typically independent of the output load. Therefore, the switching loss, particularly the switching loss associated with the input capacitance of the switch, has a significant impact on the light load efficiency of the converter.
In many applications wherein the synchronous rectifier is required to drive heavy loads, the active rectifier circuit may not be adequate to efficiently convey the required current. Conventionally, circuit designers have advantageously parallel-coupled a number of active rectifier circuits to increase the current capacity of the synchronous rectifier. At light loads, however, the switching losses attributable to the additional rectifier switches further reduce the efficiency of the power converter.
Accordingly, what is needed in the art is a synchronous rectifier topology capable of reducing the switching loss associated with the input capacitance of the rectifier switches, particularly as the output load is reduced, thereby increasing the efficiency of the converter.